The measured Born cross sections are listed in
Table REF . The ratios of the Born
cross sections for the neutral and charged \(e^+e^-\rightarrow \pi \pi h_c\)  modes
are also listed in Table REF ; the cross sections for the
charged channel are taken from Ref. [1]},
where vacuum polarization effects were not taken into account.
A corresponding correction factor \((1+\delta ^v)\)  is applied to the previous Born cross section.
The common systematic uncertainties in the two measurements cancel in the ratio calculation.
The combined ratio \({\cal R}_{\pi \pi h_c}\)  is obtained with a weighted
least squares method [2]} and determined to be
\((0.63\pm 0.09)\) , which is within 2\(\sigma \)  of the expectation of isospin symmetry, 0.5.
